1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus, and more particularly to a mechanism for preventing deviation in writing position of a mechanism that performs optical scanning simultaneously on a plurality of image carriers.
2. Description of the Related Art
Image forming apparatuses such as a copying machine, printer, facsimile machine, and printing machine of, for example, electrophotographic type are equipped with an optical scanning device as a device that forms an electrostatic latent image according to an original or image information onto a photosensitive element that is used as a latent image carrier.
Aside from the formation of monochromatic images, some optical scanning devices are used to form images in a plurality of colors to obtain a full-color image.
In recent years, a method that allows simultaneous exposure scanning on a plurality of photosensitive elements has often been used in color laser printers, color digital copying machines, and the like so as to improve productivity.
An optical scanning device capable of such simultaneous exposure scanning on a plurality of photosensitive elements has scanning optical systems corresponding to the respective photosensitive elements. The number of optical elements needed increases simply in proportion to the number of photosensitive elements, with an inevitable increase in parts count.
As a mechanism for simultaneously performing exposure scanning on a plurality of photosensitive elements, an optical scanning device of so-called opposed scanning type has already been known which uses only one rotating deflection unit which is an expensive part. Optical elements are arranged generally symmetrically about the rotating deflection unit so that both the right and left sides are scanned with a plurality of independent light beams (for example, see Japanese Patent Application Laid-open No. 2008-76586).
Japanese Patent Application Laid-open No. 2008-76586 discloses a configuration of the optical scanning device of so-called opposed scanning type which can perform exposure scanning on both the right and left sides by using the single rotating deflection unit.
Such an optical scanning device of opposed scanning type typically has a plurality of scanning optical systems, i.e., as many as the number of photosensitive elements in one optical housing in order to perform exposure scanning on each individual photosensitive element.
For space savings of the device, the only rotating deflection unit has upper and lower, two stages of reflecting surfaces. Independent scanning optical systems are provided for the respective stages.
The optical elements that constitute a scanning optical system include a scanning lens, a plurality of reflecting mirrors, and a long lens (toroidal lens) that has power in a sub-scanning direction. The layout of such optical elements and the performance of the elements themselves have a significant impact on the image quality.
For example, in an optical scanning device that is capable of simultaneous exposure scanning on a plurality of photosensitive elements, it is important to always adjust deviations of the scanning lines formed on the respective photosensitive elements (here, the curvature of the scanning lines) to constant values. With uneven geometry, the scanning lines can cause misregistration resulting in image deterioration.
In particular, when a color image forming apparatus or the like develops toners of different colors on a respective plurality of photosensitive elements, color deviations on the photosensitive elements can cause a significant deterioration in color reproducibility.
The curvature of the scanning lines is typically dominated by the scanning lenses that have power in the sub-scanning direction (typically corresponding to long lens), provided in the scanning optical system.
More specifically, a scanning line curves when the focal lines of the scanning lens which form the center of the optical axis are not in parallel with the mounting surface (seating surface) of the lens. The curvature of the focal lines is inevitable because of the process limitation in lens formation. The curvature of the focal lines could be reduced but with an increase in the fabrication cost.
Resin lenses have been used more often recently due to advantages such as low price and the formability of free curves in particular. Resin lenses suffer the curvature of the focal lines more severely than glass lenses do, because of internal distortion during molding, uneven mold temperatures, etc.
In such an optical scanning device of opposed scanning type, it is also important that all the beam spots on the respective photosensitive elements have equal characteristics. Even a slight difference in the characteristics of the beams on the respective photosensitive elements can produce image defects such as poor color reproducibility and hue variations.
As employed herein, the “beam spot characteristics” include not only the beam spot diameter but also the beam intensity (light intensity) and the beam spot position (imaging position). What is essential is how to perform uniform exposure onto each photosensitive element.
Examples of the causes for the deterioration of the beam spot characteristics include a deformation of the mounting positions of the optical elements due to thermal expansion and a deviation in the incident positions of the beams on the respective optical elements because of elevated temperatures inside the optical housing of the optical scanning device.
To prevent the occurrence of such deviations in the beam incident position and suppress color deviations, the foregoing Japanese Patent Application Laid-open No. 2008-76586 discloses the following configuration.
That is, a scanning optical device includes: a plurality of light sources; a light source holding member that holds the light sources; a deflection scanning unit that deflects light beams emitted from the light sources in a scanning manner; a plurality of scanning optical systems that are arranged on only one side or both sides of an axis of rotation of the deflection scanning unit, and scan photosensitive elements with the respective different light beams that are deflected in a scanning manner by the deflection scanning unit; a housing member that accommodates the light source holding member, the deflection scanning unit, and the plurality of scanning optical systems; and a biasing unit that biases the light source holding member against the housing member in a direction of the axis of rotation of the deflection scanning unit, the plurality of scanning optical systems including light beam reflecting units that reflect the respective light beams, any one of the light beams being reflected by at least a plurality of light beam reflecting units. In the scanning optical device, the positions of the plurality of scanning optical systems with respect to the axis of rotation of the deflection scanning unit, the number of light beam reflecting units, and the biasing direction of the biasing unit can be set to reduce the amount of variation in the relative position between the irradiation positions of the photosensitive elements with the respective light beams when the light source holding member tilts against the biasing force of the biasing unit.
Now, suppose that the deflection scanning unit in the optical scanning device is driven by a motor or the like. In such a case, heat from the motor can sometimes reach the optical members in the peripheral areas through air flows created by the rotation of the deflection scanning unit. The foregoing resin lenses, if used, may cause deviation of the optical axis due to variations in the temperature distributions in the direction of the optical axis and in the main-scanning direction.
To prevent such a thermal deformation of the resin lenses, there has been proposed a configuration to enclose the deflection scanning unit in a shielded space independent of the resin lenses (for example, see Japanese Patent Application Laid-open No. 2008-96952).
The configuration for suppressing thermal deformation of the resin lenses, disclosed in Japanese Patent Application Laid-open No. 2008-96952, can promise to avoid an increase in the ambient temperature of the resin lenses and other optical elements because of the air flows. Such a configuration, however, gives rise to the following new problem.
Each component of the optical scanning device is supported on the housing of the optical scanning device, and there is no means to prevent heat propagation through the housing to the supporting section of each component.
Consequently, the positions of the scanning lines can vary due to variations in the mounting positions of the optical elements and changes in the mounting orientations because of thermal deformation of the light source holding member and thermal deformation of the housing caused by heat sources in the optical scanning device (such as when driving the rotating deflection unit). In particular, when the opposed optical paths on the right and left of the rotating deflection unit undergo symmetric changes, the irradiation positions of the photosensitive elements with the respective light beams move in opposite directions, causing even severer color deviations. Such a problem has been unsolved yet.